Automotive-type CO and NOx catalysts are guaranteed for at least 50,000 miles for OEM catalytic converters, and 25,000 miles for less expensive replacement converters. During this period of use, the catalyst is exposed to repeated high-low temperature cycling, sometimes reaching nearly 1,000.degree. C., and, when mis-used, even exceeding the melting point of the ceramic base. Under the best conditions, over time the active surface area of the noble metal bearing washcoat coatings of such monoliths or pellets decreases significantly thereby causing a gradual loss of catalytic efficiency.
Several million spent catalytic converters are currently scrapped annually, each containing about one to two grams of platinum, palladium, and rhodium catalyst. Such mixtures, since the early 1980's, are typically in the ratio of Pt =1, Pd =0.4, and Rh =0.1. Prior to the early eighties only platinum, or platinum together with palladium was used. In view of the cost and relative scarcity of these noble metals, their recovery is a virtual necessity.
Commonly, the catalyst used for the emission control of automotive exhaust comprises a cordierite ceramic monolith of low surface area onto which has been deposited a high surface area washcoat of alumina and small amounts of other inorganic oxides. The alumina provides a high surface area substrate for the adsorption thereon of the catalytic noble metals. The small amounts of non-alumina base metal oxides in the washcoat act as stabilizers to surface area loss at high temperature and/or as catalytic promoters. Cordierite ceramic used for monoliths and pellets contains low surface area alumina, silica, and magnesia as its main constituents. The washcoat is usually between about 10 and 30 percent of the monolith by weight. Typically, the noble metal content of the coated monolith is 0.1 to 0.3 percent of the monolith by weight.
Though relatively few process details are published, a high temperature inductively-generated plasma melting process for catalyst monolith scrap at approximately 1,800.degree. C. appears to be more economical than previous multi-step aqueous aqua regia leaching methods.
The plasma process is both capital- and energy-intensive. It relies on melting the entire catalyst to recover a small fraction of one percent of the catalyst mass.
The multiple steps of the aqueous leaching process are expensive complications which are compounded by the need for environmentally acceptable disposal of the large quantities of the acidic waste solutions generated. Rhodium (which is known to be refractory to aqueous aqua regia leaching) is the most difficult to recover, and in practice, only 50% is typically recovered from concentrates containing platinum and palladium even at a total noble metal concentration of up to 5 wt. %.
In yet another prior process, spent monolith scrap is added to commercial copper smelters. The noble metals end up in the copper, from which they are later separated. Only a limited amount of the monolith scrap can be added to the smelter at a given time, as excessive amounts can interfere with slag separation operations. There is considerable dissatisfaction with the commercial use of all of the above noble metal recovery methods.
Specifically with respect to the prior art, attention is directed to U.S. Pat. No. 2,828,200 which describes the removal of noble metals from composites which involves the treatment thereof with gaseous aluminum trichloride at elevated temperatures and then recovering the gaseous products. More specifically an alumina-platinum catalyst may be reacted with aluminum trichloride formed by vaporization of aluminum trichloride crystals. In the article entitled "Recovering Platinum-Group Metals from Auto Catalysts" by James E. Hoffmann, Journal of Metals, June 1988, pages 40ff, a variety of leaching techniques are described for recovering the noble or precious metals from each catalyst.
Leaching techniques and other processes of types described above are mentioned in the review article entitled "Noble Metal Recovery From Spent Automotive Catalysts" by Michael J. D'Aniello, Jr., Society of Automotive Engineering Paper No 820187 (1981).
In an Abstract of German Patent Document 2,659,390 platinum recovery from spent catalyst uses fluorine treatment to form platinum fluoride which is then decomposed. The Abstract of Japanese Patent Document J5 0087-919 describes recovery of platinum from waste catalysts utilizing a dry gas containing carbon monoxide and chlorine in a single step process. A similar teaching is found in the Abstract of Japanese Patent Document J5 0087-920.
The Abstract of Belgian Patent 812 171 describes palladium recovery from spent catalysts by heating the catalyst with an organic chloride compound and then condensing the resulting vapor.
German Patent 2,415,069 describes the use of 9% CCl.sub.14, in 91% air for the recovery of palladium only.
The Abstract of Japanese Patent Document J8 0044-139 which appears to be equivalent to Japanese Patent Document J5 0123 026 describes platinum group metal recovery from spent catalysts by treating in an atmosphere containing carbonyl chloride. The Abstract of Japanese Patent Document J8 0043-060 which appear to correspond to Japanese Patent Document J5 0123 025 describes palladium recovery from spent palladium catalysts by treatment with gas mixtures containing carbon monoxide and chlorine, again in a single stage.
The Abstract of Japanese Patent Document J5 0087-921 is similar in its use of chlorine and carbon monoxide. The Abstract of Japanese Patent Document J5 1123 723 describes the recover of platinum group metals from spent catalyst by incorporating aluminum therein and heating the product in a chlorine containing gas stream to recover the metal as the chlorides.